The present invention relates to a method and apparatus for resonance testing of materials and structural components. The test body or sample and certain components of the testing apparatus form a system capable of oscillating and the system is driven or excited in resonance. The testing apparatus comprises a load mechanism operated by a pressure medium and a servo-hydraulic control unit operatively connected to the load mechanism.
Resonance testing of materials and structural components is employed for many purposes, especially for material strength testing and durability or useful life testing. Such resonance testing methods take advantage of the fact that strong forces or rather dynamic loads may be generated by a relatively low energy input in a spring mass system driven at its resonance frequency, whereby the forces are applied as testing loads to the test sample or body. The test sample constitutes the spring of the oscillatory system, which spring must take up the forces occurring when the system oscillates. The mass of the oscillatory system comprises, for example, the piston of a hydraulic load mechanism, the piston rod, the clamping means for the testing sample, and possibly also auxiliary masses participating in the oscillation. When such a spring mass system is operated at its resonance frequency, it is merely necessary to supply to the system the power required to make up for the power losses resulting from the damping caused by the test sample and other friction and damping losses of the oscillatory system to maintain the oscillation. Further, it is possible to control the force applied to the test sample by varying the power supplied to the system to thereby vary the oscillation amplitude. The resonance frequency f of such a system may be ascertained, as is well known, from the spring rate and from the mass of the system: ##EQU1## By varying the spring rate c and/or the mass m it is possible to vary the resonance frequency f of the system.
German Patent Publication No. 2,213,736 describes a method for performing resonance testing with the aid of a material testing machine comprising a main piston cylinder arrangement for producing static or slowly variable loads and a further, smaller piston cylinder arrangement connected to the main cylinder for producing dynamic loads at resonance. The main cylinder produces substantially a mean force or a preload static force, whereas the second cylinder acts as an exciter for the dynamic alternating load. The known apparatus also comprises hydraulic servo-valves for the control of the pressure medium supplied to the two cylinders as well as electrical control and regulating devices. The servo-valves permit a very rapid and exact control of the pressure medium supply and removal from the load cylinders. Accordingly, a respectively rapid and precise control of the load is accomplished through these valves. The known apparatus further includes pressure storage means arranged for cooperation with the main cylinder providing the static load. These pressure storage means act as equalization or compensation containers for the pressure medium. These pressure medium storage containers receive or supply the pressure medium quantities which are moved along as a result of the oscillating movements of the static load applying piston, when an additional preload is applied through the static load applying, so called preload piston during resonance operation. Generally, such a known apparatus may be used to generate static loads as well as variable loads having any desired load characteristic, for example, for random testing. The apparatus may also be used to apply dynamic loads in the resonance operation.
The oscillatory system of the apparatus according to said German Patent Publication comprises the test sample or body, a mass secured to the piston rod of the main cylinder, as well as the pistons and the piston rods of the main piston cylinder arrangement and of the exciter piston cylinder arrangement. The remainder of the known testing machine is substantially rigid. For producing the oscillatory load, the double-acting exciter cylinder is supplied with pressure medium through a hydraulic servo-valve in such a manner that both cylinder chambers receive the pressure medium in an alternating manner. Thus, each stroke movement of the piston supplies an exciting energy to the oscillating system through the piston of the exciter cylinder. The direction of the force applied to the pressure medium and the direction of the piston are respectively the same. Thus, this excitation or rather repeated excitation maintains the once started oscillation. The desired load may be adjusted by adjusting the oscillation amplitude at a respective control and regulating unit.
When the exciter piston is at its upper or at its lower return point, the pressure in the chambers of the exciter cylinder is rather small, because the pressure ratios in the chambers are reversed as a result of each piston stroke. However, the supply pressure of the pressure medium supplied for the excitation is very high. As a result, respective throttle losses must be taken into account in the pressure medium supply. Therefore, the exciter cylinder for producing the dynamic load is dimensioned as small as possible so that only small pressure medium quantities are required and that high testing frequencies may be achieved.
The direct control of the pressure medium in the preload or main cylinder during resonance operation would require high driving powers for the hydraulic drive means due to the large pressure medium quantities. Further, large valve units with large flow areas would be required for the same reason. These characteristics of the prior art apparatus thus have substantial control and regulating disadvantages. The pressure medium quantities necessary for the operation of a hydraulic cylinder may be roughly circulated from the oscillation amplitude or the piston stroke and the testing frequency.
Another disadvantage of the known apparatus is seen in that in addition to the main piston cylinder arrangement, an exciter piston cylinder arrangement is required together with the hydraulic and electric control means for producing the dynamic loads including switching devices for the several types of operation as well as pressure medium storage means which need to be switched on and off. These additional structural requirements result in a complicated structure, which for that reason alone is rather expensive and which makes the testing procedure rather involved.
Normally, in connection with hydraulic testing procedures, it is assumed that the employed pressure medium is incompressible for calculating purposes. Stated differently, the pressure medium is assumed to be rigid and not elastic, although in fact it has a certain elasticity. Generally, the eleasticity of the pressure medium is undesirable and it has disadvantageous effects, especially with regard to the control and regulating devices. Thus, the elasticity of the pressure medium has hardly been utilized heretofore. However, it is, for example, known to use the elastic pressure medium as a counter spring for oscillating masses.